In a Code Division Multiple Access (CDMA) wireless system, such as Universal Mobile Telecommunications System (UMTS), scrambling codes (SC) are used to distinguish signals transmitted from different wireless base stations.
Wireless terminals measure properties of signals received from surrounding base stations and report the measurements back to the network along with the corresponding scrambling code for each received signal. The measurements reported by wireless terminals are processed by the network. Each measurement reported by a wireless terminal is mapped to a specific and unique cell/sector of the network, using the scrambling code as an identifier of the cell/sector. This process is often called scrambling code resolution. Scrambling code resolution can be performed in real-time by the network during soft handoff using the neighbour (NB) list of the serving cell/sector, or it can be performed off-line in drive tests or call trace processing tools.
A scrambling code is the key to mapping a measurement to a cell/sector of the network. A network typically has a limited set of scrambling codes. For example, in UMTS there are 512 scrambling codes. This means that the same scrambling codes are re-used within a network. With correct network planning, there should only be one cell/sector in the area surrounding a terminal with a particular scrambling code and therefore there should no ambiguity as to which cell/sector that scrambling code corresponds to. However, in real life scenarios situations can arise where there can be more than one cell/sector in the area surrounding a terminal with a particular scrambling code and this causes ambiguity as to which cell/sector that scrambling code corresponds to. If a scrambling code is mapped to an incorrect cell/sector of the network, this can cause problems with handoff, as the network may attempt to handoff the wireless terminal to an incorrect handoff target. This could result in dropped connections.
Many scrambling code resolution strategies rely on neighbour (NB) lists. Network physical information and geometrical rules are used when measured SCs are not included in the NB list, or when the NB list is not available at all. NB list rules can be unreliable, due to missing entries in the NB list. Unable to detect and solve SC collisions. A SC collision can occur when a wireless terminal measures two different cells/sectors that have the same SC. This is typical when there is bad SC planning, which also leads to SHO failures. The use of geometrical rules can have a problem of relying on network topology information which can be incomplete or wrongly populated. For a given SC, a geometrical rule attempts to estimate the sector, having that SC, with the strongest signal level. Even if the topology information is correct, the method is impacted by propagation prediction errors and assumptions. For example, it is not always correct to assume that the closest candidate to the serving sector is the one to be received with strongest signal level. Terrain, antenna tilts, fading, building, etc. may cause that the strongest one is a further away candidate.
US 2010/0150109A1 describes a method of identification of a femtocell base station as a handover target. Each base station is configured such that it transmit a broadcast signal which has different timing information to neighbouring base stations. This is achieved either by a central control node giving different timing offsets to base stations, or by base stations setting a timing offset for themselves. Timing offsets applied to base stations are of the order of a multiple number of frames.